Voltage regulators are an essential component of most electronic devices which operate at a specified DC voltage. Typically, the electronic devices are powered with a source voltage that is fluctuating (i.e., provided by a power supply connected into a wall socket) or at an inappropriate amplitude (i.e., provided by a battery). The purpose of a voltage regulator is to convert the source voltage into the operating DC voltage of the electronic devices.
One type of a commonly used voltage regulator is a switching voltage regulator. Switching voltage regulators employ one or more switching elements and an inductor, transformer, or a capacitor as an energy storage element between the source and the load. The switching elements may be, for example, power metal-oxide semiconductor field-effect transistor (MOSFET) switches. The switching regulator regulates the voltage across the load by varying the ON-OFF times of the switching elements so that power is transmitted through the switching elements and into the energy storage element in the form of discrete current pulses. The current pulses may be generated by one-shot timers or other circuitry. The energy storage element then converts these current pulses into a steady load current so that the load voltage is regulated.
Switching regulators include control circuitry to control the ON-OFF times of the switching elements. The percentage of time that a switching element is ON is referred to as its duty cycle. The duty cycle can be varied in three ways by: (1) fixing the frequency of the pulses and varying the ON or OFF time of each pulse; (2) fixing the ON or OFF time of each pulse and varying the frequency of the pulses; or (3) varying both the ON and OFF times of each pulse and varying the frequency of the pulses (e.g., hysteretic mode control). Examples of prior art constant frequency switching regulators include the LT1307, LTC1625, and LT1074, developed by Linear Technology Corporation, of Milpitas, Calif. Examples of prior art non-constant frequency switching regulators include the MAX1710 (constant on-time), developed by Maxim Integrated Products, Inc., of Sunnyvale, Calif., the CS5120 (constant off-time), developed by ON Semiconductor, of Phoenix, Ariz., and the LT1500, LTC1148, and LTC1778 of Linear Technology Corporation.
Constant frequency switching regulators are in general preferred to non-constant frequency switching regulators, since the frequency can be selected to avoid noise-sensitive regions. For example, when using switching regulators in communications equipment such as wireless devices, it is desirable to keep the switching frequency away from the communication frequencies of the communications equipment. Constant frequency operation also enables multiple power converters to be synchronized when it becomes necessary to deliver higher power levels to the output.
However, constant frequency switching regulators are in general more complex to design, have a slower transient response, and cannot operate over as wide a range of duty cycles as non-constant frequency switching regulators. Switching regulators must be able to operate efficiently at low duty cycles and over a wide range of input and output voltages to provide the voltages required by modern electronic devices, which may be very low compared to the source voltages. With today's microprocessors requiring faster transient response and lower operating voltages than previous generations, every effort must be made to improve the transient response and increase the duty cycle range of switching regulators, while meeting cost goals.
At present, there are no switching regulators that simultaneously provide the advantages of both constant frequency and non-constant frequency operation. While constant frequency regulators suffer in the transient response and the range of operating duty cycles as compared to non-constant frequency regulators, non-constant frequency regulators may not be able to avoid the sensitive frequencies of the electronic devices and deliver high power levels to the output.
Current non-constant frequency regulators such as the MAX1710 and the LTC1778 are able to achieve approximately constant frequency operation through the use of a flexible one shot timer to control the ON-time of one of the switching elements. The one shot timer allows the switching regulators to operate at very low duty cycles and convert high input voltages to low output voltages. However, the switching frequency can still vary significantly due to second order effects in the switching regulator.
In view of the foregoing, it would be desirable to provide circuits and methods for achieving constant frequency operation with non-constant frequency switching voltage regulators.
It further would be desirable to provide circuits and methods for adjusting the switching frequency of a non-constant frequency switching regulator through the ION and VON inputs of a one shot timer used to control the duty cycle of the switching regulator.
It also would be desirable to provide circuits and methods for synchronizing multiple switching regulators to deliver higher power levels to the output.